The U.S., Europe and China are about to confront the gulf between their carbon-cutting ambitions and those of less industrialized countries.

President Barack Obama and Chinese President Xi Jinping surprised many observers last month by jointly unveiling long-range plans to curb carbon-dioxide emissions. The move followed a pledge by the other big carbon-emitting economy, the European Union, to reduce greenhouse gases as part of a new round of United Nations-led climate negotiations.